A conventional object detecting apparatus mounted on a vehicle uses a laser light, for instance, to detect a distance to an object such as a preceding vehicle. This detecting apparatus intermittently drives a laser diode to radiate the laser light towards the forward area of the vehicle, and detects the laser light reflected from the forward object by a photo sensor. The detecting apparatus measures the distance to the forward object based on a time difference between a light radiation time and a light reception time.
Specifically, as disclosed in JP 2002-031685A, the detecting apparatus comprises a light radiation unit for radiating a laser light, a polygon mirror and a light receiver unit for receiving a reflected laser light. The polygon mirror is shaped in a frustum of a hexagonal pyramid and rotatable as a scanning mirror.
According to this construction, the polygon mirror reflects the laser light radiated from the light radiation unit and directs it to the forward area of the vehicle. As the polygon mirror is rotated and the laser light from the light radiation unit is directed to each side surface of the polygon mirror, so that the angle of reflection of the laser light at the polygon mirror is adjusted to scan a predetermined range of the forward area of the vehicle by the laser light. The receiver unit includes a Fresnel lens and a light receiving device to receive the laser light reflected from the forward object and measure the distance to the object.
It is required to increase the output power of the laser light to cover a long distance or a wide area. The laser light of increased output power level is not desired so that human eyes may be protected from receiving the laser light.
In the case of a vehicle stop-and-go system which automatically stops and starts a vehicle in correspondence with a stop and start of a preceding vehicle, the output power of the laser light is decreased when the vehicle is at a stop. This is because it is more likely that the human eyes may receive the laser light when the vehicle is at a stop.
For this reason, the output power of the laser light must be controlled precisely. It is however difficult because the current-to-power characteristics of laser diodes vary from diode to diode.